The present invention relates to a sheet stacker and more particularly a sheet stacker for use with electrostatographic reproducing apparatus.
In an electrostatographic reproducing apparatus commonly in use today, a photoconductive insulating member is typically charged to a uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the usual document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with developing powder referred to in the art as toner. Most development systems employ a developer material which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive area. This image may subsequently be transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image to a support surface, the photoconductive insulating member is cleaned of any residual toner that may remain thereon in preparation for the next imaging cycle. Alternatively, the electrostatic latent image may be generated from information electronically stored or generated in digital form which afterwards may be converted to alphanumeric images by image generation, electronics and optics. In such a printer application a beam of light such as a laser beam may be used to selectively discharge the photoconductor.
The geometry of the processing component in many automatic reproducing machines is such that the copies produced have the image on the top side and sequential copies enter the collecting tray with the copy or image side up. This is satisfactory if only a single copy of a single image is desired or multiple copies of a single image is desired. In both cases no distinction between sequential copies is required and all copies may be readily collected with the image side up. It is also satisfactory if the original documents fed to the copying machine are fed in reverse order, last or bottom sheet first and first or top sheet last. In this instance, the collected set has the top sheet face up on top and the bottom sheet face up on the bottom of the set. However, in most instances of copying sets of documents, the set is face up with top sheet on the top of the set and if copying according to normal procedures feeding the top document first, the top document number one, is copied producing a copy face up and set so produced has sheet number one face up on the bottom of the set and the last sheet face up on the top. In addition in electronic printing it is advantageous to be able to print from the first page to the last page in order since if you print from the last page to the first page the substance of the first to the last pages must be stored in the printers memory thereby increasing the size, cost and complexity of the memory required.
These difficulties may be avoided in the reproduction of successive sheets of a set by inverting each copy or print in the final set as it is collected face down with the top sheet on the bottom and the bottom sheet on the top. The Xerox 3700 copier and 2700 printers are examples of commercial applications of such inverters.
Typically, such inverters occupy a large amount of space which when accompanied with a high capacity stacker capable of stacking up to about 2,000 individual prints increases the overall machine volume required which when associated with the smaller low volume copiers and printers undesirably increases the size and thereby decreases customer acceptability.